Frogeye leaf spot (FLS) is a fungal disease named for its signature visual pattern: an eye-like circular or angular lesion on leaves of infected plants. The small spots of this soybean leaf disease may not appear threatening at first glance, but each spot not only impairs the plant’s growth but can also potentially spread the infection. If left unchecked, it can spread to pods and seeds and cause substantial yield loss. Infected residue can overwinter in your field, leading to potential losses the following year.
In 2024, FLS was responsible for the loss of an estimated 3 million soybean bushels.¹
Properly identifying soybean frogeye leaf spot is critical to set it apart from other soybean foliar diseases and address it accordingly.
Frogeye leaf spot disease cycle
Frogeye leaf spot is caused by the fungus Cercospora sojina. Like many fungal infections, it tends to infect immature plants, including seedlings, and senescing flowers. An FLS infection can begin in several ways:
- Infected seeds at planting: Frogeye leaf spot can become obvious early, as weaker, infected plants begin to grow — or fail to germinate altogether.
- Residual infection: When infected soybean residue decomposes, it releases new spores, a catalyst for new infections when warmer, humid conditions arrive.
- Weather: Splashing rain, irrigation water or wind can also disperse spores over short distances.
Cercospora sojina has a polycyclic infection cycle, meaning it can release spores several times in a single growing season. Temperatures between 70 F and 80 F with humidity from frequent rainfall or irrigation causes the pathogen to produce spores.² An infected plant becomes a host for the fungus and a potential source of infection for nearby plants. Tillage can bury residual material, and the fungus could resurface in two or three years to spark another infection.
Scouting for frogeye leaf spot
If your field has a history of soybean frogeye leaf spot, there is a high probability you’ll find it again this season, especially if fungicide treatments were not applied.
Symptoms typically become visible anytime from R1 to R6. Prioritize scouting for frogeye leaf spot around R1. Scout at least a few plants in every 10 acres.
Frogeye leaf spot symptoms
Frogeye leaf spot appears as spots measuring 1 to 5 mm in diameter, in a burst scattered across the surface of leaves as though resulting from rainfall. In rare cases, lesions can also be found on stems.
As the infection takes hold, the spots grow and the centers of the spots turn pale gray, while their borders take on a red, brown or purple color. In the middle of the pattern, you may see small black dots resembling a pupil in the “eye” of the spot. These are the spores of the fungus.
Environmental factors contributing to frogeye leaf spot
Watch for frogeye leaf spot following hot, humid summers. The amount and severity of the disease can differ greatly between different areas of your field due to environmental conditions. For example, rows that stay wet longer from early morning shade, dew or irrigation create a more hospitable habitat for the frogeye leaf spot fungus to grow.
Not every leaf spot can be blamed on FLS, but if the spot has a purplish border, FLS is the likely cause. Harder-to-diagnose frogeye leaf spot symptoms can also develop on stems and pods, as lesions that start out reddish and darken to brown and develop a sunken center with age. The fungus that causes this disease can even invade the pod walls, ultimately reaching and damaging the seed, further perpetuating the disease cycle as infected seed used for seed production can spread the pathogen when planted.
Experts are available to help you manage frogeye leaf spot and other soybean leaf diseases. Reach out to your seed retailer, a nearby extension office agent, or a seed company professional like your regional BASF representative.
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Endnotes
- “Crop Protection Network.” Crop Protection Network, 14 Mar. 2025, cropprotectionnetwork.org/publications/soybean-disease-loss-estimates-from-the-united-states-and-ontario-canada-2024. Accessed 19 Jan. 2026.
- “Frogeye Leaf Spot of Soybean.” NDSU Agriculture, 20 Mar. 2025, www.ndsu.edu/agriculture/extension/publications/frogeye-leaf-spot-soybean. Accessed 19 Jan. 2026.
